Rima Ariadaeus is a fine example of a straight rille. Ariadaeus Rille
is over 300 km in length; a portion of the central section of the rille
about 120 km in length is pictured here. A linear section of the crust
is dropped down along parallel faults or breaks in the crust to form a
graben or fault trough. The ridges crossing the trough and the surrounding
plains units have been offset by the trough, proving that they are older
than the faults. Some craters are cut off by the faults and are, therefore,
older. Other craters lie on the wall of the trough and are younger than
the faulting. The faulting must be relatively young because so few craters
appear to be younger than the faults, and because the edges of the trough
appear to be crisp and little affected by slumping and other mass wasting.

There is a gradation between straight rilles, gently curving rilles,
and sinuous rilles modified by volcanic flows. This example shows no trace
of associated volcanism; it is, therefore, considered to be the end member
of the sequence, where only pure faulting is involved.-H.M.